Modern electronic prepress operations utilize laser scanning systems to write or record images for subsequent reproduction or to scan a prerecorded image at a predefined resolution rate. Such scanning systems may write or record images or scan prerecorded images on various prepress media including, photo or thermal sensitive paper or polymer films, photo or thermal sensitive coatings or erasable imaging materials mounted onto an image recording surface or photo or thermal sensitive paper, polymer film or aluminum base printing plate materials, all used in electronic image reproduction. Such media are mounted onto a recording surface which may be planar but which is more typically curved and scanned with a recording or scanning beam. The primary components of such a system include a recording surface, usually a drum cylinder and a scan mechanism disposed and movable within the drum cylinder. The system also includes a processor, with an associated storage device, for controlling the scanning mechanism and for scanning a prerecorded image, a photodetector and detector processor. The processor and associated storage device may be housed within the system itself or separate from the system with appropriate interconnection to the system.
The processor, in accordance with stored programming instructions, controls the scanning mechanism to write or read images on the plate or other medium mounted to the inner drum cylinder wall by scanning one or more optical beams over the inside circumference of the drum cylinder while the drum cylinder itself remains fixed.
The scanning and hence the recording are performed over only a portion of the cylinder inner circumference, typically between 120.degree. and 320.degree. of the circumference of the drum cylinder. The optical beam(s) are typically emitted so as to be parallel with a central axis of the cylinder and are deflected, by for example, a spinning mirror, Hologon or Penta-prism deflector so as to form a single scan line or multiple scan lines which simultaneously impinge upon the recording surface. The deflector is spun or rotated by a motor about an axis of rotation substantially coincident with the central axis of the drum cylinder. To increase the recording speed, the speed of rotation of the beam deflecting device can be increased. To even further increase the recording speed, multiple beam scanning has been previously proposed.
One such proposed multiple beam scanner has utilized a spinning dove prism with a single light source, as discussed, for example, in U.S. Pat. No. 5,214,528. Using a dove prism beneficially allows the use of a multiple beam source, e.g. a laser diode array, while eliminating the need for multiple beam correction elements and associated hardware. Additionally, for reasons which need not be discussed here, the scan speed of multiple beam systems using a dove prism can exceed that of other types of proposed multi-beam systems.
U.S. Pat. No. 5,097,351 proposes another type of multi-beam scanning system which utilizes a controlled piezo-reflecting mirror, or what might be better characterized as a wobbling mirror, in lieu of a dove prism. In this system, each of two laser beams follow separate optical paths. Each optical path has focusing and collimating lenses and an acousto-optical modulator (AOM). Hence, the proposed system requires both AOMs and a wobbling mirror. The controlled wobble reflector is disposed in only one of the optical paths, i.e. the wobbling mirror reflects only one of the two beams, and is driven to redirect, e.g. rotate, the reflected beam in synchrony with the rotation of the spin deflector. Errors in the direction of the redirected beam are detected, and corrected by driving the wobbling mirror to adjust angular alignment during recording operations.
It is of primary importance that the multiple light beams contact the spin deflector as close as possible to a desired location to ensure that the appropriate scan lines are formed on the recording surface and hence the desired image is properly recorded. This includes maintaining the desired spacing or overlapping relationship of the simultaneously scanned beams with respect to each other and the reduction or elimination of any differential scan line bow between successive scan lines.
A wobble, for example in the spinning dove prism of the '528 patent or degradation in performance due to wear in moving components of the support structure, will cause a misalignment of the element with respect to the spin deflector and can create significant banding artifacts in the scan lines which repeat every two scan passes. The effect of misalignment on system banding can be reduced, for example, by increasing the ratio of the beam diameter at the spinning prism to the beam diameter at the spin reflector. However, alignment errors, can also create a twinning between groups of the multiple beams. This is because in the proposed systems the spin prism rotates only half a turn for each full turn of the spin deflector. Accordingly, if a misalignment exists, the multiple beam system is restricted to recording during only every other rotation of the spin deflector to obtain high quality results. A four beam system is accordingly only two times faster than single beam system, an eight beam system only four times faster, and so on.
Further, in multi-beam systems at least one of the light beams must be redirected in synchrony with the rotation of the spin deflector. Any synchronization errors between the redirection of the redirected beam(s) and the angular position of the spin deflector will make it impossible to obtain a proper scan of the recording surface and result in improper or unsatisfactory recording of the image. Small changes in the phase locking of spin or wobble element motion and spin deflector rotation can create banding groups.
If the multiple beams leaving the dove prism described in the '528 patent or the beam reflected from the wobbling element described in the '351 patent drift or flutter relative to the spin deflector, due for example to small motor speed variations or wobble mirror driver inaccuracies, the cross-scan spacing between the multiple scan lines may change, one or more scan lines may bow and/or scan lines may become non-parallel even to the point of intersecting, and this will create visible artifacts, e.g. banding. The '351 patent, as can best be understood, proposes a technique for correcting synchronization errors between the described wobbling mirror and the spin deflector using a quad detector and feedback arrangement to control the wobbling mirror to adjust the motion of the wobbling mirror to correct for synchronization errors.
The above reference related 08/687,931 application discloses an improved multi-beam scanning system which includes a spinning element, such as a dove or wedge prism in conjunction with a correction element, such as described in the above referenced related 08/687,928 application, to correct beam alignment errors in a spin element of the type described in the '528 patent. The above referenced 08/687,931 application also discloses improved techniques for correcting synchronization errors between the rotation of the spin or wobble element and the spin deflector. Accordingly, the inventions described in the above referenced related applications, which are incorporated herein in their entirety by reference, can be utilized to improve on previously proposed systems, such as those disclosed in the '528 patent, by correcting misalignments in the spin element, and hence the misalignment of the beam being rotated by the spin element with respect to the spin deflector. Additionally provided are improved techniques for correcting synchronization errors between the spin or wobble element and the spin deflector of the '528 or '351 patents. However, the system described in the 08/687,931 application requires a spin element as well as a correction element such as a translating lens.